Effect of bleed hole on flow and heat transfer performance of U-shaped channel with dimple structure

Internal and external cooling methods are mostly combined in gas turbine blades where serpentine channels are more usual for internal cooling. Dimple structure is an effective heat transfer augmentation approach on coolant channel due to its considerable advantage on pressure penalty. Mass extraction by bleed hole on the coolant channel surface is inevitable to realize the film cooling. Therefore, the objective of this study is to numerically analyze the effect of bleed extraction on a U-shaped internal passage with dimple structures. RANS and LES methods are both adopted as the numerical model to present detailed flow and heat transfer data. The investigated rectangular U-shaped channel consists of two passes and a 180° turning bend with a height-to-width ratio of 2. Different bleed hole positions and installation angles are considered to obtain a relative optimal structure in this paper. Results indicate that heat transfer rates are higher in second pass than those in the first pass with significant heat transfer enhancement by dimple. Bleed hole extraction will alter the flow characteristics inside dimple cavity and between dimple arrays resulting in higher heat transfer rates. Comparing different bleed hole cases, cases when bleed holes are installed inside dimple cavity and near windward edge correspond to obviously higher heat transfer rates than other cases. For the pressure penalty, the dimpled channel with or without bleed possesses significant smaller friction ratio than typical ribbed channel. The thermal performance of dimpled channel with bleed hole is comparable to typical rib and pin-fin channel. Better performance on friction ratio and thermal performance will be obtained after the bleed extraction effect.